With just weeks to go untilChina's18th Communist Party Congress, a number of critical voices have been locked up or put under house arrest, while several leading rights activists have been ordered to leave Beijing or face being confined to their homes or placed under surveillance.

Those affected range from well-known human rights campaigners and lawyers to anonymous petitioners who claim they have been imprisoned to stop them travelling to protest in Beijing during the Congress, which begins on November 8.

Activists and academics say the tightening of security aims to ensure that while there will be no shortage of lengthy speeches inside Beijing's Great Hall of the People, where the Congress will be held, outside very little will be said at all.

"The basic idea is really that anybody who could cause trouble has to be kept under control," said Eva Pils, an associate professor and director of the Centre for Rights and Justice at the Faculty of Law of the Chinese University of Hong Kong. "It is a very tense period. Many people will be under some pressure."

Prof. Pils said security officials' methods went from "pestering text messages that purport to be caring but are really intimidating" to "house arrest".

In Shanghai, activists say at least ten petitioners have been locked up since late September, among them Wang Kouma, an anti-corruption campaigner, and Tong Guojing, a housing petitioner. Relatives and friends said both were taken into custody on September 25.

"It's certainly to do with the 18th Congress," said Mr Tong's wife, Zhang Fangxian, showing a document given to her by local security officials explaining her husband had been imprisoned on charges of "gathering a crowd to disturb social order".

Ms Zhang said her husband had been taken away by two uniformed officers and five plainclothes police at around 9am on September 25.

"I would really hope he will be released after the Congress is over. But I was told by some friends yesterday that after 30 days of criminal detention, my husband will probably get an other two-and-a-half years in a labour camp."

Wu Xuewei said his wife, the petitioner Mao Hengfeng, had been arrested in Beijing on September 30 and was now in a Shanghai detention centre.

Asked if he believed the arrest was related to the Congress, he said: "Everyone knows it, but just can't say it. You can easily get into trouble for saying something."

Cai Wenjun, a third Shanghai petitioner, said "at least ten" petitioners had been detained.

"It is obvious that this [crackdown] is related to the 18th Congress," she said. "Quite a few people are missing too."

She claimed security agents were targeting petitioners who were trying to travel to Beijing to air their grievances during the Congress.

"The plain clothes are all over the place at Shanghai's train stations, as well as the train stations in nearby cities. They catch one when they see one. We don't know where these caught petitioners are sent to, probably 'black jails'," she added, referring to the notorious holding centres that human rights groups describe as "illegal prisons".

China's massive security apparatus has also stepped up a gear in the capital, Beijing, placing outspoken activists under house arrest or pressuring them into temporary exile.

Hu Jia, a well-known rights activist and democracy campaigner, said he had been under house arrest since September 18 and claimed security officials had instructed him to "delete all your online posts, or be responsible for whatever consequences".

"[This is] all to 'clear the ground' for the Party Congress, and when their threats and violence are not working, they put strict limitations on my personal freedom," he said.

Wang Lixiong, an influential writer and democracy activist, said he was still in Beijing but would consider leaving "for a few days" if the situation worsened.

"I have been told that they wish I would go somewhere else during the 18th Congress," he said.

Mr Wang said his wife, the Tibetan writer Woeser, had already departed. In a blog posted on the internet last week, Woeser described herself as "a special person required to be outside of Beijing before the opening of the 18th Party Congress."

Li Jun, a social activist, said his website had been blocked and he had left Beijing to "dodge" the Congress. "Police called and sounded extremely nervous" he said. "I sensed the tension in the air."

Prof. Pils, from the Chinese University of Hong Kong, said one Chinese lawyer had told her he had received "a barrage of texts messages" from security agents.

"They say things like: 'Take it easy in the next couple of weeks' or, 'Be careful', or, 'You've been working too hard – take a rest.'"

"It certainly signals that they are very impatient for people to be gone, to be off their patch, out of their particular sphere of responsibility and clearly the focus has to be on the bigger cities and in particular Beijing."

Prof. Pils said a number of activists had simply decided to leave to avoid being placed under surveillance or house arrest. "If somebody is willing to travel somewhere, say to a tourist spot, then that is OK. You might also try to go to your hometown or to go away on business but I understand that some people just find themselves 'house-arrested'."

An official from Shanghai's Public Security Bureau said they could not comment on the petitioners' arrests without approval from the city's Communist Party propaganda department. Calls to the propaganda department went unanswered, as did a written request for information to security authorities.